TRUE REPTILES. 219 
Fic. 263.—Skeleton of the tortoise, from below. 7, joints of the backbone 
grown together ; 7, ribs formed into a solid cover ; s#, shoulder-bones ; 
, hip-bones covered by carapace, which has grown over them ; #/, plas- 
tron, or under cover. 
the limbs are adapted to aquatic or terrestrial life, as the 
case may be. About forty species are known in North 
America, north of Mexico. 
Marine Turtles (Chcloniide),— This family has a 
wide distribution in warm and tropical seas; five spe- 
cies are known. 
The leather tur- 
tle, or Sphargis 
(Fig. 264), is 
the rarest and 
largest known. 
The shell is a 
thick, leathery 
skin, composed 
of six longitudi- 
nal plates, form- 
ing raised ridges. Fic. 264,—Leather turtle (Sphargzs). 
